If price is a factor, such as cost for Rx's, then I would take a look at both Graupner options (X-8N or X-8E):
REVIEW: Graupner X-8N HoTT (4ch) 2.4Ghz Radio System - Radios & Servos @ URC Forums
I've been running the X-8N for about a year and a half and they offer a 2 year warranty which is the best in the industry. For the price/features, I think they hit the mark for the quality you get. I have used Graupner's customer service and received top notch service. I have used other high end systems in the past such as
Radiopost and
KO Propo, but they left me disappointed. Radiopost customer service is almost non existent and while KO Propo customer service is top notch, I ran into too many intermittent problems with the system for me to continue maintaining long term.
While I agree that Futaba is an excellent choice, I feel they are a pinch on the pricey side which has attracted me to Graupner and I have no regrets with that decision, especially because they have a US based service/repair center. There are few other options with that luxury... here are some other systems to consider as well in your search:
Which surface radio system is the best? <Updated 24-DEC-2014> - Radios & Servos @ URC Forums
I've heard mixed reviews for HiTec where some Rx's tend to overheat in nitro's running 45min+ mains
Airtronics recently went out of business and they used to be the US distributor for Sanwa radios... it's not clear if anyone has re-opened a service/repair center in the US for Sanwa since Airtronics shut down.
So for me, the only two brands I would personally consider are Futaba or Graupner... when my 2 year warranty expires this coming December, I plan to upgrade to the X-8E which several other folks are running in my area and I think the extra features are worth the slightly higher price
KO Propo has recently released a new system, so it's possible they may have worked out the kinks? If so, then I would highly recommend the Dasmikro Rx's which are KO Propo compatible and will offer a reasonable price for replacement Rx's.